1. Field of the Invention
The invention relates to purge gas protected, pressurized, solid oxide electrolyte, fuel cell generator modules, with a gas feed system effective to react with unreactive fuel at shutdown, and to an array of such generator modules for use with a variety of auxiliary components in a power generation system of 100 kW to 50 MW plus capacity.
2. Background Information
Fuel cell based, electrical generator apparatus utilizing solid oxide electrolyte fuel cells (xe2x80x9cSOFCxe2x80x9d) arranged within a housing and surrounded by insulation are well known, and taught, for example, by U.S. Pat. No: 4,395,468 (Isenberg); U.S. Pat. No. 5,169,730 (Reichner); U.S. Pat. No. 5,750,278 (Gillett et al.); U.S. Patent Application Publication No.: 2002/0110716A1 U.S. Ser. No. 09/784,610 (Holmes et al. -Docket No. 0E 7684 US), and xe2x80x9cSolid Oxide Fuel Cellxe2x80x9d, Westinghouse Electric Corporation, pp 1-12, October, 1992. Tubular type fuel cells can comprise an open or close ended, axially elongated, self-supporting, ceramic tube air electrode material, completely covered by thin film ceramic, solid electrolyte material. The electrolyte layer is covered by cermet fuel electrode, except for a thin, axially elongated, interconnection material.
Flat plate type fuel cells can comprise a flat array of electrolyte and interconnect walls, where electrolyte walls contain thin, flat layers of cathode and anode materials sandwiching an electrolyte. xe2x80x9cCorrugatedxe2x80x9d plate type fuel cells can comprise a triangular or corrugated honeycomb array of active anode, cathode, electrolyte and interconnect materials. Other fuel cells not having a solid electrolyte, such molten carbonate fuel cells are also well known, and can be utilized in the article and method of this invention.
Development studies of SOFC power plant systems have indicated the desirability of pressurized operation. This would permit operation with a coal gasifier as the fuel supply and/or use of a gas turbine generator as a bottoming cycle. Integration is thought commercially possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow.
Conventional combustor in a gas turbine system typically exhibit high nitrogen oxides (NOx) emissions, combustion driven oscillations and instabilities, excessive noise and low efficiencies. Although significant advances have been made to mitigate these problems, it has proved difficult to design a practical, ultra-low NOx, high-turn-down ratio combustor due to poor flame stability characteristics. The combination of all the above factors results in pressurized SOFC generator module design being suitable as a replacement of conventional gas turbine combustor and applicable to more efficient combined cycle power plants required to meet increasingly stringent emission targets.
A variety of fuel cell uses in power plant systems are described in the literature. In U.S. Pat. Specification No. 3,972,731 (Bloomfield et al.), a pressurized fuel cell power plant is described. There, air is compressed by compressor apparatus, such as a compressor and turbine which are operably connected, which is powered by waste energy produced by the power plant in the form of a hot pressurized gaseous medium, such as fuel cell exhaust gases. These exhaust gases are delivered into the turbine, which drives the compressor for compressing air delivered to the fuel cells. In U.S. Pat. Specification No. 5,413,879 (Domeracki et al.) a pressurized SOFC is also integrated into a gas turbine system. There, pre-heated, compressed air is supplied to a SOFC along with fuel, to produce electric power and a hot gas, which gas is further heated by combustion of unreacted fuel and oxygen remaining in the hot gas. This higher temperature gas is directed to a topping combustor that is supplied with a second stream of fuel, to produce a still further heated gas that is then expanded in a turbine.
U.S. Pat. No. 4,622,275 (Noguchi et al.) also describes a fuel cell power plant, where reformed, reactive fuel if fed to an anode of the cell, an expansion turbine connected to a compressor feeds compressed gas into the cathode of the cell, which compressed gas is mixed with anode exhaust gas which had been combusted.
Fuel cell pressurization, while advantageous in system performance, presents several practical difficulties to the SOFC generator designer, two of which are: (1) The pressure boundary must be able to withstand pressures up to 20 atmospheres; (2) Because fuel and air are brought together within the SOFC generator, care must be taken to avoid the potential of an unstable condition during startup, operation, and especially during regular and emergency shutdown. For atmospheric operation, the expected explosive overpressure would be about 115 psi (792.4 kPA) which existing designs can accommodate by mechanical strength alone. However, the expected explosive overpressure at 20 atmospheres is about 2315 psi (15950 kPA). A protective containment system to prevent the accumulation of an explosive gas mixture is required.
Many of these problems were solved by U.S. Pat. Specification No. 5,573,867 (Zafred et al.). There, a separate source of a purge gas of, for example air, argon or nitrogen was fed to a purge gas volume between the SOFC modules and the outer containment pressure vessel housing the modules. Separate air and gaseous fuel sources were fed to the SOFC modules. The large amount of compressed air required use of large air manifolds passing through the purge air volume and into the air electrodes of the modules. Zafred et al. recognized that a purge gas space was necessary to control any unreacted fuel gas flow from the modules by dilution with the purge gas.
An even more serious problem can occur, if, for some reason, the gas turbine generator system shuts down in an emergency situation, or even just for normal servicing. What results is that the sole source of SOFC feed air from the turbine generator""s compressor is cut off, with possible unreacted fuel still remaining in the SOFC modules, especially if there is an unexpected emergency.
This problem could be addressed by providing a plurality of pressurized, auxiliary air sources, such as bottled pressurized air or a high pressure auxiliary air compressor feeding into the normal low pressure auxiliary air source, as described in S. E. Veyo xe2x80x9cThe Solid Oxide Fuel Cell Technology of the Siemens Westinghouse Power Corporationxe2x80x9d presentation slides 25-33, Apr. 27, 2001, Milan, Italy. There, for a 220 kWe SOFC/turbine generator system, pumped auxiliary air and pressurized tank auxiliary air were used as back-up air sources. The air could be fed by piping into the fuel cells, or fed by separate piping through the SOFC pressure vessel to a duct-burner, to mix with exhaust and fresh fuel to power a gasifier. This solution would take up substantial space, require complicated valving and pumping means exterior to the SOFC module""s pressure vessel, a sophisticated control system, and in the case of a separate, stand alone, emergency compressor, would be quite costly.
What is needed is an inexpensive source of high pressure emergency auxiliary air available to be fed into the air inlets of the SOFC modules in the correct proportions to burn unreacted fuel, eliminating the possibility of an explosive fuel-air gas mixture being generated during depressurization. What is also needed is a simple source of high pressure air that can substitute for a stand alone high pressure auxiliary air compressor. Therefore, it is one of the main objects of this invention to provide an inexpensive and simple means to supply highly pressurized air to pressurized SOFC modules in a shut-down situation, in the correct air fuel proportions, to react/burn any unreacted fuel. It is another main object of the invention to simplify the entire feed arrangement relating to pressurized air, especially at emergency shut-down.
These and other objects of the invention are accomplished by providing a fuel cell generator apparatus characterized by containing at least one fuel cell assembly module containing a plurality of fuel cells, each fuel cell having electrolyte between an air (oxidant) electrode and a fuel electrode; a module housing enclosing the module; a pressure vessel having two ends surrounding the module housing, such that there is an air accumulation space between the module housing and the pressure vessel; where the pressure vessel has fuel gas inlet tubing connecting to a module fuel gas inlet; an air compressor associated with a gas turbine generator system for supplying compressed air; a compressed air inlet connecting to the air accumulation space where air occupies the air accumulation space and then flows to a module air inlet; and an exhaust gas outlet connecting to a module exhaust gas outlet; where the air accumulation space provides an air accumulator with sufficient volume to control any unreacted fuel gas flow from the module by dilution with air and to react with any unreacted fuel gas within the module upon shutdown of the apparatus, such as upon shut-down of the air compressor. The volume rates of free fuel:free air within the fuel cell generator apparatus is from about 1:3-6, that is from about 1:3 to from about 1:6.
The fuel cells are preferably tubular solid oxide electrolytic fuel cells and they will generally operate at temperatures over about 650xc2x0 C., up to about 1100xc2x0 C. The module housing and the fuel cells operate in the xe2x80x9cpressurizedxe2x80x9d mode, that is over at least about 2 atmospheres, or about 28.5 psi (pounds per square inch 196.4 kPA), and preferably at about 2 to 10 atmospheres. The air compressor of an associated gas turbine-generator will be the sole source of air/oxidant to the fuel cell generator. During operation, after depressurization, a small air blower supplies air to the cells during cooling. In all cases it is essential that the compressed air be at a higher pressure than the fuel gas; usually from 0.5 atm to 5.0 atm (50.6 kPA to 506 kPA) higher so no fuel can enter the purge gas space.
The invention also resides in a method of operating a fuel cell generator apparatus characterized by: (1) passing fuel gas, through a fuel inlet and into a plurality of fuel cell assembly modules, each module containing a plurality of fuel cells, each fuel cell having electrolyte between an air (oxidant) electrode and a fuel electrode, where the modules are each enclosed by a module housing and where the module housings are surrounded by an axially-elongated pressure vessel having two ends, such that there is an air accumulation space between the module housings and the pressure vessel, the fuel gas also passing through the pressure vessel enclosing the modules and through a tube within the air accumulation space to the fuel inlet; (2) passing pressurized air from an air compressor associated with a gas turbine generator through the pressure vessel into the air accumulation space to circulate within the air accumulation space, where the air then passes into the module through an air inlet, where the oxidant gas dilutes any unreacted fuel gas flow from the module and where the air accumulation space provides an air accumulator with sufficient volume to react with any unreacted fuel gas; and (3) passing exhaust gas and any unreacted fuel gas out of the pressure vessel; where the accumulated air will react with any unreacted fuel gas if the air compressor is shut-down. Preferably the pressure vessel is tubular.
The generator apparatus can operate at interior temperatures up to 1100xc2x0 C. in a flow of fuel, and oxidant such as air. The generator apparatus will have associated with it and will be working in cooperation with well known auxiliaries, such as controls; an oxygen or air pre-heater; a fuel gas compressor; a fuel desulfurizer; an air compressor which may by operably connected to a power turbine coupled to an electric generator; a source of fuel gas; heat exchangers; a heat recovery unit to recover heat from the hot fuel cell exhaust gases; and a topping combustor, to provide an electrical power generation system. This type power system could be, for example, part of an integrated, coal gasification/fuel cell-steam turbine combination power plant, featuring a plurality of coal gasifiers and fuel cell generator arrays or power blocks with associated DC/AC conversion switchgear. This type power system could also be part of a natural gas fired combustion turbine system, or the like.
This pressurized fuel cell generator apparatus design provides a unique safety feature for fuel cell operation, and is an easily transportable assembly. By combining several modules within the pressure vessel, it is possible to operate the fuel cells at high pressure, thus greatly improving overall SOFC module voltage, efficiency and power output to the extent that it becomes feasible to integrate this apparatus with an industrial gas turbine in a high efficiency combined cycle power plant. This integration is possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow. In other words, the SOFC module acts as a conventional combustor in a gas turbine and it provides the volumetric flow rate, at the required temperature and pressure, which is discharged through the turbine.
Integration of these pressurized SOFC modules with conventional gas turbines in a combined cycle power plant, can boost overall electrical efficiencies to 65% to 70%, providing values presently unmatched by any other power generation technology. The pressurized SOFC generator modules will result in a design that can be used with the full range of existing commercial combustion turbines and will not require major modification to the units.